A major problem of the coating and finishing industry, both in terms of raw material usage and in environmental effects, concerns the solvent components of paint. In a spray coating application of a resinous material, the resinous material is typically dissolved in an organic solvent provided with a viscosity suitable for spraying. This is required because it has been found that at each stage of the process for atomizing and conveying a resinous material in liquid form to a substrate, the liquid resists high speed deformation. Organic solvents are added to the resinous liquid because they have the effect of separating the molecules of resinous material and facilitating their relative movement making the solution more deformable at high speeds and therefore more susceptible to atomization. Substantial effort has been expended to reduce the volume of liquid solvent components in preparing high solids coating compositions containing about 50 percent by volume of polymeric and pigmentary solids. Nevertheless, most high solids coating compositions still contain from 15 to 40 percent by volume of liquid solvent components.
The problem with such a high volume content of liquid solvents in coating compositions is that during handling, atomization or deposition of the coating compositions, the solvents escape and can become air contaminants if not properly trapped. Once the coating composition is applied to a substrate, its solvents escape from the film by evaporation and such evaporated solvents can also contaminate the surrounding atmosphere. Additionally, since most solvents react with oxidants, pollution problems of toxicity, odor and smog may be created. Attempts at overcoming such environmental problems have proven to be costly and relatively inefficient.
It has previously been proposed in Cobbs U.S. Pat. No. 4,247,581 to reduce solvent content in paint by mixing a liquid or gas blowing agent into the paint to produce an easily atomized foam solution just upstream from the discharge outlet of a dispensing device. Rehman et al U.S. Pat. No. 4,630,774 disclosed an improvement of this concept wherein a foaming chamber and turbulence inducing device was incorporated into the coating dispenser to better control the formation of the foam prior to discharge from the outlet of the dispenser. U.S. Pat. Nos. 4,505,406; 4,505,957; and, 4,527,712 also disclose methods and/or apparatus for mixing liquid or gas blowing agents into paint formulations to reduce solvent content. Each of these patents are owned by the assignee of this invention.
More recently, U.S. Pat. No. 4,923,720 to Lee et al disclosed a method and apparatus for the continuous production of a coating material formulation in which a substantial amount of the liquid solvent component is removed and replaced with a supercritical fluid such as supercritical carbon dioxide which functions as a diluent to enhance the application characteristics of the coating material formulation. The supercritical carbon dioxide and some liquid solvent material, e.g., about two thirds less than is required in other coating compositions, are intermixed with polymeric and pigmentary solids to form a coating material solution or formulation having a viscosity which facilitates atomization through an airless coating dispenser. As the coating material formulation is discharged from the dispensing device(s) toward a substrate, the supercritical carbon dioxide "flashes off" or vaporizes to assist in atomization of the high solids coating composition and to reduce drying time of the composition on a substrate. This type of coating material formulation, and those types of foamable formulations described above, have the advantage of substantially reducing the adverse environmental affects caused by high solvent content.
It has been observed that in order to produce a coating material solution or formulation with the desired application characteristics, the relative proportion of the liquid coating composition and supercritical fluid should be maintained at a predetermined ratio or within a predetermined range. This produces a formulation which is either "single phase" or "multiple phase". A formulation is considered in single phase when the supercritical fluid is dissolved or dispersed within the liquid coating composition forming a single continuous phase of material having a given composition and density. A formulation is considered to be multiple phase when two or more phases of material are present, each having a different composition and density. A single phase formulation can be converted to a two phase formulation by adding more supercritical fluid, or by reducing system pressure, so that the first phase is generally continuous and the second phase is typically a "dispersed" phase or one having bubbles dispersed in the first phase.
One problem with systems of the type disclosed in the Lee et al U.S. Pat. No. 4,923,720, which are designed for the continuous production of a coating material formulation, is an inability in some instances to control and/or maintain the relative proportion of liquid coating composition and supercritical fluid so that either a single phase or a multiple phase formulation is formed for transmission to coating dispensers. The ineffectiveness of the Lee et al system in this respect can be attributed to the pumping system it employs for supplying the liquid coating composition and/or supercritical fluid to the coating dispensers, and to the loss of supercritical fluid within the system caused by leakage, uneven mixing and the like. No provision is made in the Lee et al system for monitoring the liquid coating composition and/or supercritical fluid content within the system, nor are there any means for selectively altering the volume of either of these components in the course of the system operation.
Many of these problems are also present in an alternative, batch-type system disclosed in U.S. Pat. No. 5,009,367 to Nielsen, which is owned by the same assignee as the Lee et al Pat. No. 4,923,720. As shown in FIG. 6 of the Nielsen patent, the liquid coating composition and supercritical fluid are each weighed prior to introduction to the system, in predetermined proportions, and then introduced into a loop where they are intermixed within a static mixer in preparation for transmission to coating dispensers. This weighing procedure is cumbersome, and, like the Lee et al system, no provision is made in the Nielsen apparatus for adjusting the relative proportion of liquid coating composition and supercritical fluid once the system begins to operate.
U.S. patent application Ser. No. 07/662,401, filed Feb. 27, 1991, and U.S. patent application Ser. No. 07/728,051, both entitled "Method and Apparatus For Forming And Dispensing Single and Multiple Phase Coating Material Containing Fluid Diluent", which are owned by the assignee of this invention, has addressed problems presented with systems of the type disclosed in the Lee et al U.S. Pat. No. 4,923,720 and Nielsen U.S. Pat. No. 5,009,367. The method and apparatus disclosed in application Ser. Nos. 07/662,401 and 07/728,051 includes a control system which is effective to adjust the supply of supercritical fluid and/or liquid coating composition in accordance with variations in a sensed parameter, such as capacitance of the coating material formulation, in order to (1) maintain the coating material formulation in substantially single phase or in substantially multiple phase, as desired, in the course of an operating run; and, (2) to ensure that the desired ratio of liquid coating composition to supercritical fluid is obtained before the formulation is supplied to coating dispensers for deposition onto a substrate. The control system disclosed in application Ser. Nos. 07/662,401 and 07/728,051 is highly effective in forming and maintaining a coating material formulation having the desired supercritical fluid and liquid coating composition content, and provides substantial flexibility in accommodating different types of liquid coating compositions and different application characteristics on a given substrate.
One potential limitation of the of system of Ser. Nos. 07/662,401, and 07/728,051 however, are that they provide more control capability than may be required for certain applications. There has therefore been a need for a simpler, less expensive system of forming coating material formulations of the type described above for certain applications. A second limitation of this system is that it is relatively time consuming to clean in the event a change of color of the formulation is desired, and thus may not be readily adaptable or practically usable for multiple color applications.